Windows - XBMC on 2 displays at the same time

I have 2 LCD-TVs connected to my HTPC, one in the living room, one in the sleeping room.
I only use one display at a time.
My problem is, that I cannot duplicate desktop in Windows, I don't know why....so XBMC is either displayed on LCD 1 OR LCD 2.

Not sure what version of windows you have... Windows 7 & 8 use the DisplaySwitch.exe command and the parameters allow clone. (Win key +P ) brings up a GUI switch screen. Other versions, would be according to the graphic display capabilities of the card you have, so if you have two output ports on your gfx hardware, it's likely you can have two displays cloned.

As a side note: Cloning is possible with different resolutions supported and is pretty common with 1920 x 1200 cloned 1920 x 1080 it's the 1080 dimension that gets stretched to 1200 pixels. I suppose if the primary & secondary was switched it might be the reverse, but I haven't confirmed that. As baijuxavior noted most of this is highly dependant on the graphic card in place.

This would be a limitation of the graphic card or system you are using (obviously you would need 3 separate individual ports) and the graphic drivers supplied by the manufacturer. You'll have to do your own homework on this, look at the links I left in depth, many gamers use a 3 monitor set-up.